The MAT locus on chromosome III is Saccharomyces cerevisiae controls the mating type of the cell. Two other loci, HML and HMR contain MAT sequences but are not usually transcribed. This transcriptional repression is regulated by four genes, sirl-4; recessive mutations in any one of which can activate the normally silent HML and HMR. Messenger RNA's coding for the genetically characterized genes of MAT are transcribed divergently from the center of the MAT locus. Since this sequence is present both of the silent loci, HML and HMR, and MAT, the DNA sequences initially recognized by the SIR gene products must lie a kilobase or more from the start of transcription. This phenomena is similar to the position effect observed in Drosophila and more recently at the hemoglobin locus in humans in which deletions can activate normally silent genes. This proposal, an investigation of the position effect at HML and HMR, consists of two parts: (1) I will use a combination of in vitro mutagenesis and yeast transformation to locate DNA sequences of HML and HMR that are required for SIR mediated repression. Mutations that allow the normally silent genes to become active will identify possible sites of interaction between the SIR gene products and HML or HMR. (2) The mechanism for the position effect may involve the nucleoprotein structure at HML and HMR. I will use nucleases to probe the chromatin structure of the mating type genes. Using a combination of genetics and chromosome structural analysis, I hope to uncover the mechanism of the position effect of HML and HMR. The mechanism of mating type position effect might be relevant to similar phenomena in other eukaryotes.